1. Field of the Invention
This invention relates generally to a load sensing hydraulic system and more particularly to a pilot control circuit incorporated within the load sensing system.
2. Description of the Prior Art
The load sensing variable displacement pump of a load sensing hydraulic system usually has a pressure responsive displacement controller which automatically adjusts pump output to meet flow and pressure demands of the system. If none of the hydraulic motors of the hydraulic systems are being operated in a manner to generate a load pressure signal for transmission to the displacement controller, the output of the pump is reduced to a minimum level sufficient to maintain the system pressure at a relatively low margin pressure. The hydraulic motors of many such load sensing systems are controlled by a pilot operated control valve which is moved to an operating position by directing pressurized pilot fluid to the appropriate end of the control valve through a manually operated pilot control valve. Since the margin pressure is normally 30 at a sufficient level to support pilot actuation of the pilot operated control valves, the pilot circuit of some of those systems utilize the load sensing variable displacement pump as a source of pressurized pilot fluid. To prevent over-pressurization of the pilot system, a pressure reducing valve is commonly provided in the pilot circuit to maintain the pressure of the pilot circuit at a level slightly less than the margin pressure of the main system.
One of the problems encountered with such systems occurs when a pilot operated directional control valve is moved to a position sufficient to allow a load supported by a hydraulic motor to freefall such that the side of the motor receiving fluid from the directional control valve tends to cavitate. During a freefall condition, the flow requirements to fill the expanding or intake side of the motor is usually greater than the output capacity of the pump even though the pump strokes to its maximum displacement setting. Thus the intake side of the motor tends to cavitate and since the main supply conduit from the pump is connected with the cavitated side of the motor through the control valve, the main system pressure drops below the pressure level of the pilot circuit. Consequently, the pressure level of the pilot circuit drops allowing the control valve to move back to its neutral position prematurely stopping the lowering of the load.
The present invention is directed to overcoming one or more of the above problems.